Source and Drain Stressors with Recessed Top Surfaces

ABSTRACT

An integrated circuit structure includes a gate stack over a semiconductor substrate, and a silicon germanium region extending into the semiconductor substrate and adjacent to the gate stack. The silicon germanium region has a top surface, with a center portion of the top surface recessed from edge portions of the top surface to form a recess. The edge portions are on opposite sides of the center portion.

BACKGROUND

Metal-Oxide Semiconductor (MOS) devices are key components of integrated circuits. The performance of MOS devices affects the performance of the entire integrated circuits in which the MOS devices are located. Therefore, methods for improving the performance of the MOS devices have been studied.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIGS. 1A through 9 are cross-sectional views and top views of intermediate stages in the manufacturing of Metal-Oxide Semiconductor (MOS) devices in accordance with some exemplary embodiments.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the embodiments of the disclosure are discussed in detail below. It should be appreciated, however, that the embodiments provide many applicable concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are illustrative, and do not limit the scope of the disclosure.

Reduction in the size and the inherent features of semiconductor devices (e.g., Metal-Oxide Semiconductor (MOS) devices) has enabled continued improvement in speed, performance, density, and cost per unit function of integrated circuits over the past few decades. In accordance with a design of the MOS devices and one of the inherent characteristics thereof, modulating the length of a channel region underlying a gate between a source and drain of a MOS device alters a resistance associated with the channel region, thereby affecting a performance of the MOS device. More specifically, shortening the length of the channel region reduces a source-to-drain resistance of the MOS device, which, assuming other parameters are maintained relatively constant, may allow an increase in the current flow between the source and drain when a sufficient voltage is applied to the gate of the MOS device.

To further enhance the performance of MOS devices, stress may be introduced into the channel region of a MOS device to improve carrier mobility. Generally, it is desirable to induce a tensile stress in the channel region of an n-type MOS (“NMOS”) device in a source-to-drain direction, and to induce a compressive stress in the channel region of a p-type MOS (“PMOS”) device in a source-to-drain direction.

An available method for applying compressive stresses to the channel regions of PMOS devices is growing SiGe stressors in the source and drain regions. Such a method typically includes the steps of forming a gate stack on a semiconductor substrate, forming spacers on sidewalls of the gate stack, forming recesses in the silicon substrate along the gate spacers, epitaxially growing SiGe stressors in the recesses, and annealing. Since SiGe has a lattice constant greater than that of silicon, it expands after annealing and applies a compressive stress to the channel region, which is located between a source SiGe stressor and a drain SiGe stressor.

A process for forming Metal-Oxide-Semiconductor (MOS) devices with stressors is provided in accordance with various exemplary embodiments. The intermediate stages of forming the MOS devices are illustrated. The variations of the embodiments are discussed. Throughout the various views and illustrative embodiments, like reference numbers are used to designate like elements.

FIG. 1A illustrates substrate 20, which is a portion of wafer 10. Substrate 20 comprises a first portion in device region 100 and a second portion in device region 200. Substrate 20 may be a bulk semiconductor substrate such as a silicon substrate, or may have a composite structure, such as a Silicon-On-Insulator (SOI) structure. The material of substrate 20 may include silicon, silicon carbon, or the like. Gate stacks 122, which are parallel to each other, are formed in region 100 and over substrate 20. Gate stacks 222, which are parallel to each other, are formed in device region 200 and over substrate 20. In some embodiments, neighboring gate stacks 122 are spaced apart from each other by uniform distances S1, and neighboring gate stacks 222 are spaced apart from each other by uniform distances S2, although distances between neighboring gate stacks 122 (or 222) may also be non-uniform. Each of gate stacks 122 and 222 includes gate dielectric 24 and gate electrode 26. Gate dielectrics 24 may comprise silicon oxide or a high-k material having a high k value, for example, higher than about 7. Gate electrodes 26 may include doped polysilicon, a metal or a metal alloy, a metal silicide, or the like. Hard masks 28 may be formed over gate stacks 122 and 222, wherein hard masks 28 may comprise silicon nitride, for example.

FIG. 1B illustrates a top view of the structure in FIG. 1A, wherein the cross-sectional view of device region 100 in FIG. 1A is obtained from the plane containing line A-A in FIG. 1B, and the cross-sectional view of device region 200 in FIG. 1A is obtained from the plane containing line B-B in FIG. 1B. Device regions 100 and 200 have different pattern densities and/or different sizes of gate stacks. For example, distance S1 between neighboring gate stacks 122 in region 100 may be greater than distance S2, which is the distance between neighboring gate stacks 222 in region 200. In some embodiments, distance S1 is greater than 100 nm, and width W2 is smaller than 100 nm. Ratio S1/S2 may be greater than about 2, for example.

Also, active region 123 and 223 in device regions 100 and 200, respectively, are defined by Shallow Trench Isolation (STI) regions 25. Gate stacks 122 cross active region 123, and gate stacks 222 cross active region 223. In some embodiments, active region 123 has width W1, which is greater than width W2 of active region 223. In some exemplary embodiments, width W1 is greater than 300 nm, and width W2 is smaller than 300 nm. Furthermore, ratio W1/W2 may be greater than about 2, for example.

Referring to FIG. 2, Lightly Doped Drain/source (LDD) regions 130 and 230 are formed, for example, by implanting a p-type impurity into substrate 20. Gate stacks 122 and 222 and the overlying hard mask 28 act as implantation masks, so that the inner edges of LDD regions 130 and 230 are substantially aligned to the edges of gate stacks 122 and 222, respectively. The LDD implantation may be performed using energies in a range between about 1 keV and about 10 keV, and a dosage in a range between about 1×10¹³/cm² and about 1×10¹⁶/cm². It is appreciated, however, that the values recited throughout the description are merely examples, and may be changed to different values. The LDD implantation may be performed with a tilt angle, with the tilt angle smaller than about 30 degrees, for example. Alternatively, the LDD implantation is performed vertically. In addition, pocket regions (not shown) may be formed, for example, by implanting an n-type impurity such as arsenic, phosphorous, or the like into substrate 20. The pocket implantation may be performed using energies in a range between about 20 keV and about 80 keV, and a dosage in a range between about 1×10¹²/cm² and about 1×10¹⁴/cm². The pocket implantation may be tilted. In some embodiments, the tilt angle of the pocket implantation is in a range between about 15 degree and about 45 degrees.

Referring to FIG. 3, gate spacers 34 are formed on the sidewalls of gate stacks 122 and 222. In some embodiments, each of gate spacers 34 includes a silicon oxide layer (not shown) and a silicon nitride layer over the silicon oxide layer, wherein the silicon oxide layer may have a thickness in a range between about 15 Å and about 50 Å, and the thickness of the silicon nitride layer may be in a range between about 50 Å and about 200 Å. In alternative embodiments, gate spacers 34 include one or more layers, each comprising silicon oxide, silicon nitride, silicon oxynitride, and/or other dielectric materials. The available formation methods include Plasma Enhanced Chemical Vapor Deposition (PECVD), Low-Pressure Chemical Vapor Deposition (LPCVD), Sub-Atmospheric Chemical Vapor Deposition (SACVD), and other deposition methods.

As also shown in FIG. 3, an isotropic etch is performed to form openings 136 and 236 in device regions 100 and 200, respectively. The isotropic etch may be a dry etch, wherein the etching gas may be selected from CF₄, Cl₂, NF₃, SF₆, and combinations thereof. Depth D1 of opening 136 and opening 236 may be in a range between about 150 Å and about 500 Å, for example.

Next, as shown in FIG. 4, a wet etch may be performed to expand openings 136 and 236, The wet etching may be performed, for example, using Tetra-Methyl Ammonium Hydroxide (TMAH), a potassium hydroxide (KOH) solution, or the like. In some exemplary embodiments, the TMAH solution has a concentration in a range between about one percent and about 30 percent. During the wet etch, the temperature of the TMAH may be in a range between about 20° C. and about 100° C. After the wet etching, facets are formed in openings 136 and 236, which facets may include (111) planes of substrate 20. In some exemplary embodiments, after the wet etching, depth D2 of opening 136 and opening 236 may be in may be in a range between about 20 nm and about 60 nm, for example.

After the wet etch, a pre-clean may be performed, for example, using an HF-based gas or a SiCoNi-based gas. The pre-clean may remove the silicon oxide that is formed as a result of the nature oxidation of the exposed surfaces in openings 136 and 236. A high-temperature baking may also be performed, although the baking may also be skipped. The high-temperature baking may be performed with or without the presence of HCl gas. The baking temperature may be between about 700° C. and about 900° C. The pressure of the baking gases may be in a range between about 10 Torr and about 200 Torr. The baking duration may be in a range between about 30 seconds and about 240 seconds, for example. The high-temperature baking may also remove the native oxide on the exposed surfaces of substrate 20, which exposed surfaces are in openings 136 and 236.

As shown in FIG. 5A, a semiconductor material, such as silicon germanium (SiGe), is epitaxially grown in openings 136 and 236 through Selective Epitaxial Growth (SEG), forming epitaxy regions 138 and 238 in recesses 136 and 236 (FIG. 4), respectively. Epitaxy regions 138 and 238 act as the source and drain regions and the source/drain stressors for the respective MOS devices. Throughout the description, epitaxy regions 138 and 238 are also referred to as SiGe regions. Top surface 138A of epitaxy region 138 may be higher than the interface between gate stack 122 and the respective underlying top surface of semiconductor substrate 20, and top surface 238A of epitaxy region 238 may be higher than the interface between gate stack 222 and the respective underlying top surface of semiconductor substrate 20. The process gases for the epitaxy may include H₂, N₂, Dichloro-Silane (DCS), SiH₄, GeH₄, and/or the like. The temperature of the epitaxy may be in a range between about 600° C. and about 900° C. The pressure of the process gases may be in a range between about 10 Ton and about 200 Ton. In some embodiments, an etching gas such as HCl may also be added to promote the selective growth on the exposed surfaces of substrate 20, but not on dielectrics such as gate spacers 34. The epitaxy is performed until recesses 136 and 236 (FIG. 4) are fully filled. During the epitaxy, desired impurities may be doped while the growth proceeds. For example, when boron is to be doped, B₂H₆ may be included in the process gases. Epitaxy regions 138 and 238 may have a germanium percentage in a range between about 15 percent and about 60 percent, for example, although different germanium percentages may also be used.

Due to different distances S1/S2 (FIG. 1B) and/or different widths W1/W2, the top surface profile of epitaxy regions 138 may be different from the top surface profile of epitaxy regions 238. In some embodiments, the top surface 138A of epitaxy region 138 has two edge portions 138A1 that are substantially flat. Top surface 138A of epitaxy region 138 also includes center portion 138A2, which is between two portions 138A1, wherein portion 138A2 is recessed to be lower than portions 138A1, forming recesses 139. In some embodiments, portion 138A2 include two substantially straight (planar) portions tilting to opposite directions, wherein the substantially straight portions form straight lines 29 in the cross-sectional view in FIG. 5A. Portion 138A2 form angle α with a horizontal plane that is parallel to the (major) top surface of substrate 20. In some embodiments, angle α is in a range between about 5 degrees and about 45 degrees. The lowest point of top surface portion 138A2 is lower than top surface portions 138A1 by height difference ΔT1, which may be in a range between about 1 nm and about 10 nm. Difference ΔT1 may also be the height difference between the lowest point of center portion 138A2 and the highest points of top surface 138A.

Although FIG. 5A illustrates that the bottom portion of center portion 138A2 is the joint of two straight lines 29, in alternative embodiments, the lowest portion of top surface portion 138A2 is a substantially flat surface. The recess 139, which is formed by recessed top surface portions 138A2 has a lengthwise direction parallel to the lengthwise directions of gate stacks 122, which recess 139 is schematically illustrated in FIG. 5B, which is a top view of the structure in FIG. 5A.

Top surfaces 238A of epitaxy regions 238 do not include recesses at the centers, and the center portions may be substantially flat. The difference in the top surface profile of epitaxy regions 138 and the top surface profile of epitaxy regions 238 may be generated and enlarged by adjusting the process conditions for the epitaxy. For example, increasing the flow rate of HCl may result in the generation and the increase in height difference ΔT1 (FIG. 5A). In some exemplary embodiments, the flow rate of HCl is in a range between about 50 sccm and about 250 sccm. The flow rate of DCS may be in a range between about 50 sccm and about 350 sccm, and the flow rate of GeH₄ may be in a range between about 100 sccm and about 950 sccm. It is realized, however, that the top surface profiles of epitaxy regions 138 and 238 are also affected by the sizes (such as S1, S2, W1, and W2 in FIG. 1B), the pattern densities of the patterns, and other factors. Accordingly, the optimum process conditions for generating and the desirable top surface profiles of epitaxy regions 138 and 238 may be found through experiments.

After the formation of SiGe regions 38, capping layers 144 and 244 are formed over epitaxy regions 138 and 238, respectively, through epitaxy, as shown in FIG. 6. Capping layers 144 and 244 have a low germanium percentage that is lower than the germanium percentage of epitaxy regions 138 and 238. In some embodiments, the germanium percentage of capping layers 144 and 244 is smaller than about 15 percent. Capping layers 144 and 244 may be pure silicon layers with no germanium comprised therein, or substantially pure silicon layers with, for example, less than 2 percent, or 1 percent, germanium. Accordingly, capping layers 144 and 244 are alternatively referred to as silicon caps throughout the description. Capping layers 144 and 244 may be in-situ doped with p-type impurities with the proceeding of the epitaxy, or not in-situ doped. In the embodiments that no p-type impurity or substantially no p-type impurity is doped during the epitaxy of SiGe layers 138, 238, and capping layers 144 and 244, a p-type impurity implantation may be performed to form source and drain regions for the respective MOS device.

Next, referring to FIG. 7, hard masks 28 (refer to FIG. 6), if any, are removed, and replacement gates are formed to replace gate dielectrics 24 and gate electrodes 26 (FIG. 6) in accordance with some embodiments. In alternative embodiments, gate dielectrics 24 and gate electrodes 26 are not replaced with replacement gates. In the embodiments replacement gates are formed, gate dielectrics 24 and gate electrodes 26 (FIG. 6) act as dummy gates that are removed. FIG. 7 illustrates an exemplary structure including the replacement gates. The formation process may include forming Inter-Layer Dielectric (ILD) 46, performing a CMP to level the top surfaces of ILD 46 with the top surface of gate electrodes 26 (or hard mask 28, if any), and removing the dummy gates. A gate dielectric layer and a gate electrode layer may then be formed to fill the openings left by the removed dummy gates, followed by a CMP to remove excess portions of the gate dielectric layer and the gate electrode layer. The remaining replacement gates include gate dielectrics 24′ and gate electrodes 26′. Gate dielectrics 24′ may comprise a high-k dielectric material with a k value greater than about 7.0, for example, and gate electrodes 26′ may comprise a metal or a metal alloy. ILD 46 may be formed of a dielectric material such as Phospho-Silicate Glass (PSG), Boro-Silicate Glass (BSG), Boron-Doped Phospho-Silicate Glass (BPSG), or the like. Next, contact openings 48 are formed, exposing underlying capping layers 144 and 244.

FIG. 8 illustrates the formation of source/drain silicide regions 52. Silicide regions 52 may be formed by depositing a thin layer (not shown) of a metal (referred to as silicide metal sometimes), such as titanium, cobalt, nickel, tungsten, or the like. The metal is over the devices, including the exposed surfaces of capping layers 144 and 244. An anneal is then performed to react the metal with silicon/germanium. After the reaction, a layer of metal silicide is formed between silicon and the metal. The un-reacted metal is selectively removed through the use of an etchant that attacks the metal but does not attack silicide. As a result of the silicidation, source/drain silicide regions 52 extend into capping layers 144 and 244, and may extend into SiGe regions 138 and/or 238. Alternatively, the top portions of capping layers 144 and 244 are silicided, and the bottom portions of capping layers 144 and 244 are not silicided. After the silicidation, there may be some remaining portions of capping layers 144 and 244 remaining not silicided, wherein the remaining portions of capping layers 144 and 244 are level with, and are on the opposite sides of, source/drain silicide regions 52.

FIG. 9 illustrates the formation of source/drain contact plugs 54, which are formed by filling a conductive material such as tungsten, copper, aluminum, titanium, cobalt, silicon, and/or the like, into openings 48, and performing a CMP to level the top surface of contact plugs 54 with the top surface of ILD 46. The formation of MOS transistors 160 and 260 in device regions 100 and 200 are thus finished.

The embodiments of the present disclosure have some advantageous features. Generally, large epitaxy regions (stressors) apply smaller stresses to the nearby channel regions of MOS device than small epitaxy regions since the small epitaxy regions and channels are closely located. On the other hand, the stresses applied by the large epitaxy regions cannot be increased by growing more SiGe since SiGe has significant facets, and hence excess SiGe formation will result in the landing area of contact plugs to be reduced undesirably. In the embodiments of the present disclosure, by forming recessed (dished) top surface profile for large epitaxy regions, the facets of the SiGe stressors are reduced, and hence more SiGe germanium may be grown to apply greater stresses to the respective channel regions. The device performance such as the on-currents of the respective MOS devices is improved.

In accordance with some embodiments, an integrated circuit structure includes a gate stack over a semiconductor substrate, and a silicon germanium region extending into the semiconductor substrate and adjacent to the gate stack. The silicon germanium region has a top surface, with a center portion of the top surface recessed from edge portions of the top surface to form a recess. The edge portions are on opposite sides of the center portion.

In accordance with other embodiments, an integrated circuit structure includes a semiconductor substrate, and a first and a second MOS transistor. The first MOS transistor includes a first gate stack over the semiconductor substrate, and a first silicon germanium region extending into the semiconductor substrate and adjacent to the first gate stack. The first silicon germanium region has a first top surface, with a center portion of the first top surface recessed from edge portions of the first top surface to form a recess. The edge portions are on opposite sides of the center portion. The second MOS transistor includes a second gate stack over the semiconductor substrate, and a second silicon germanium region extending into the semiconductor substrate and adjacent to the second gate stack. The second silicon germanium region has a second top surface, wherein the second top surface does not have recessed center portions.

In accordance with yet other embodiments, a method includes forming a gate stack over a semiconductor substrate, forming an opening extending into the semiconductor substrate, wherein the opening is on a side of, and adjacent to, the gate stack. An epitaxy is performed to grow a silicon germanium region in the opening, wherein the silicon germanium region has a top surface, with a center portion of the top surface recessed lower than edge portions of the top surface to form a recess. The edge portions are on opposite sides of the center portion.

Although the embodiments and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the embodiments as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, and composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. In addition, each claim constitutes a separate embodiment, and the combination of various claims and embodiments are within the scope of the disclosure. 

What is claimed is:
 1. An integrated circuit structure comprising: a semiconductor substrate; a first gate stack over the semiconductor substrate; and a first silicon germanium region extending into the semiconductor substrate and adjacent to the first gate stack, wherein the first silicon germanium region comprises a first top surface, with a center portion of the first top surface recessed from edge portions of the first top surface to form a recess, and wherein the edge portions are on opposite sides of the center portion.
 2. The integrated circuit structure of claim 1 further comprising a silicon cap over the first silicon germanium region, wherein the silicon cap is in physical contact with the first top surface.
 3. The integrated circuit structure of claim 1 further comprising: a second gate stack over the semiconductor substrate; a second opening extending into the semiconductor substrate, wherein the second opening is adjacent to the second gate stack; and a second silicon germanium region in the second opening, wherein the second silicon germanium region comprises a second top surface, and wherein the second top surface does not have recessed center portions.
 4. The integrated circuit structure of claim 3, wherein the first silicon germanium region is between the first gate stack and a third gate stack, with no additional gate stack between the first gate stack and the third gate stack, wherein the second silicon germanium region is between the second gate stack and a fourth gate stack, with no additional gate stack between the first gate stack and the fourth gate stack, and wherein a first distance between the first gate stack and the third gate stack is greater than a second distance between the second gate stack and the fourth gate stack.
 5. The integrated circuit structure of claim 3, wherein the first silicon germanium region is in a first active region having a first width, and wherein the second silicon germanium region is in a second active region having a second width smaller than the first width, and wherein the first width and the second width are measured parallel to lengthwise directions of the first gate stack and the second gate stack, respectively.
 6. The integrated circuit structure of claim 1, wherein the recess has a depth in a range between about 1 nm and about 10 nm.
 7. The integrated circuit structure of claim 1, wherein the center portion of the first top surface comprises two substantially planar portions tilting to opposite directions.
 8. An integrated circuit structure comprising: a semiconductor substrate; a first Metal-Oxide-Semiconductor (MOS) transistor comprising: a first gate stack over the semiconductor substrate; and a first silicon germanium region extending into the semiconductor substrate and adjacent to the first gate stack, wherein the first silicon germanium region comprises a first top surface, with a center portion of the first top surface recessed from edge portions of the first top surface to form a recess, and wherein the edge portions are on opposite sides of the center portion; and a second MOS transistor comprising: a second gate stack over the semiconductor substrate; and a second silicon germanium region extending into the semiconductor substrate and adjacent to the second gate stack, wherein the second silicon germanium region comprises a second top surface, and wherein the second top surface does not have recessed center portions.
 9. The integrated circuit structure of claim 8 further comprising: a first silicon cap over and contacting the first top surface; and a second silicon cap over and contacting the second top surface, wherein the first and the second silicon caps have a same germanium percentage smaller than the germanium percentage of the first and the second silicon germanium regions.
 10. The integrated circuit structure of claim 8, wherein the recess has a lengthwise direction parallel to a lengthwise direction of the first gate stack.
 11. The integrated circuit structure of claim 8, wherein the recess has a depth in a range between about 1 nm and about 10 nm.
 12. The integrated circuit structure of claim 8, wherein the center portion of the first top surface comprises a substantially straight and tilted portion, and wherein the substantially straight and tilted portion forms an angle in a range between about 5 degrees and about 45 degrees with a major top surface of the semiconductor substrate.
 13. The integrated circuit structure of claim 8, wherein the first top surface is higher than an interface between the first gate stack and a major top surface of the semiconductor substrate, and wherein the second top surface is higher than an interface between the second gate stack and the major top surface of the semiconductor substrate.
 14. The integrated circuit structure of claim 8, wherein the first silicon germanium region is between the first gate stack and a third gate stack, with no additional gate stack between the first gate stack and the third gate stack, wherein the second silicon germanium region is between the second gate stack and a fourth gate stack, with no additional gate stack between the second gate stack and the fourth gate stack, and wherein a first distance between the first gate stack and the third gate stack is greater than 100 nm, and a second distance between the second gate stack and the fourth gate stack is smaller than 100 nm.
 15. The integrated circuit structure of claim 8, wherein the first silicon germanium region is in a first active region having a first width greater than 300 nm, and wherein the second silicon germanium region is in a second active region having a second width smaller than 300 nm, and wherein the first width and the second width are measured parallel to lengthwise directions of the first gate stack and the second stack, respectively.
 16. A method comprising: forming a first gate stack over a semiconductor substrate; forming a first opening extending into the semiconductor substrate, wherein the first opening is on a side of, and adjacent to, the first gate stack; and performing an epitaxy to grow a first silicon germanium region in the first opening, wherein the first silicon germanium region comprises a first top surface, with a center portion of the first top surface recessed lower than edge portions of the first top surface to form a recess, and wherein the edge portions are on opposite sides of the center portion.
 17. The method of claim 16 further comprising: forming a second gate stack over the semiconductor substrate; forming a second opening extending into the semiconductor substrate, wherein the second opening is on a side of the gate stack; and growing a second silicon germanium region in the second opening, wherein the second silicon germanium region comprises a second top surface, and wherein the second top surface has no recess in a respective center region.
 18. The method of claim 17, wherein the growing the first silicon germanium region and the growing the second silicon germanium region are performed simultaneously.
 19. The method of claim 16 further comprising forming a silicon cap over and contacting the first top surface of the first silicon germanium region, wherein the silicon cap has a germanium percentage lower than a germanium percentage of the first silicon germanium region.
 20. The method of claim 19 further comprising, after forming the silicon cap, performing a silicidation to silicide the silicon cap. 